Wireless Local Area Networks (WLANs) are generally implemented according to a standard as defined by the ISO/IEC 8802-11 international standard (IEEE 802.11). The 802.11 standard is a standard for wireless LAN systems that operate in the 2.4-2.5 GHz ISM (industrial, scientific and medical) band or 5 GHz U-NII band. It focuses on MAC (medium access control layer) and on PHY (physical layer) protocols for so-called access point based networks and ad-hoc networks.
In access point based networks, stations within a cell will communicate directly to their associated access point. The set of stations in a cell together with the access point is called a Basic Service Set (BSS). The access point (AP) forwards messages to destination stations within the same cell or through a wired distribution system to other access points, from which such messages arrive finally at a destination station. In ad-hoc networks, the stations communicate directly to each other and there is no access point or (wired) distribution system.
The 802.11 standard supports DSSS (direct sequence spread spectrum) with differential encoded BPSK and QPSK, FHSS (frequency hopping spread spectrum) with GFSK (Gaussian FSK), and infrared with PPM (pulse position modulation). These three PHYs (DSSS, FHSS and infrared) all provide bit rates of 2 and 1 Mbit/s. Furthermore, the 802.11 standard includes extensions called 11a and 11b. Extension 11b [2] is for a high rate CCK (Complementary Code Keying) PHY, providing bit rates 5.5 and 11 Mbit/s as well as the basic DSSS bit rates of 2 and 1 Mbit/s within the same 2.4-2.5 GHz ISM band. Extension 11a is for a high bit rate OFDM (Orthogonal Frequency Division Multiplexing modulation) PHY standard providing bit rates in the range of 6 to 54 Mbit/s in the 5 GHz band.
The 802.11 basic medium access behaviour allows interoperability between compatible PHYs through the use of the CSMA/CA (carrier sense multiple access with collision avoidance) known as Distributed Coordination Function (DCF) protocol and a random back-off time following a busy medium condition. In addition, all directed traffic uses immediate positive acknowledgement (ACK frame), where a retransmission is scheduled by the sender if no ACK is received. The 802.11 CSMA/CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the point in time where collisions would most likely occur. Collisions are most likely to occur just after the medium becomes free, following a busy medium. This is because multiple stations would have been waiting for the medium to become available again. Therefore, a random back-off arrangement is used to resolve medium contention conflicts. In addition, the 802.11 MAC defines special functional behaviour for fragmentation of packets, medium reservation via RTS/CTS (request-to-send/clear-to-send) polling interaction.
The 802.11 MAC also describes the way beacon frames are sent by the AP at regular (beacon) intervals to enable stations (STAs) to monitor the presence of APs. The 802.11 MAC also includes a set of management frames, which allow a STA to actively scan for other APs on any channel available. In 802.11 AP-based networks the STAs associate to an AP with a corresponding network name or BSS identifier, normally the STAs associate with best-received and nearest AP.
Another mode of operation of 802.11 is Point Coordination Function (PCF). In this mode the medium access control is centralized. During a beacon interval, a Basic Service Set (BSS) will by turns operate in DCF mode and PCF mode as is prescribed in the 802.11 standard. Where in DCF mode both the AP and the STAs have equal access opportunities, in PCF mode the AP controls the medium access by polling the STAs. When a STA is polled, the STA is allowed to access the medium to transmit a packet. Since the AP and STAs do not perform carrier sensing and collision avoidance in the PCF mode, this may lead to unsynchronized behaviour and interference with neighbouring APs. This is evidently the case if the neighbouring APs operate on the same frequency channel as the AP and if their cells overlap with the cell of the AP. These APs will be referred to as overlapping neighbouring APs. The overlapping neighbouring APs may continuously perform collision avoidance (since the medium seems constantly occupied), resulting in unfair spatial use. Furthermore, if two or more overlapping neighbouring APs operate in PCF mode, it may lead to uncoordinated interference and thus high packet loss probabilities, due to the absence of carrier sensing and collision avoidance before medium access. One of objects of the present invention is to mitigate overlap between overlapping neighbouring APs in order to avoid the problems mentioned above.